Tricky problem from Apple interview

Welcome back!

Both Google and Apple asked an easy linked list problem. But is it really as easy as it looks? Let’s find out in this week’s coding challenge.

Today we will cover:

• Middle of the Linked List problem

• Long Polling vs WebSockets vs Server-Sent Events

Read time: under 4 minutes

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CODING CHALLENGE

Middle of the Linked List

Given the head of a singly linked list, return the middle node of the linked list.

If there are two middle nodes, return the second middle node.

Example:

Input: head = [1,2,3,4,5]
Output: [3,4,5]
Explanation: The middle node of the list is node 3.

Solve the problem here before reading the solution.

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SOLUTION

To solve this problem, we can use the two-pointer technique. We'll use two pointers: a slow pointer and a fast pointer. The slow pointer moves one step at a time, while the fast pointer moves two steps at a time.

When the fast pointer reaches the end of the list, the slow pointer will be at the middle node.

This is because the fast pointer travels twice as fast as the slow pointer. So, when the fast pointer reaches the end, the slow pointer will be halfway through the list.

The time complexity of this solution is O(n), where n is the number of nodes in the linked list.

SYSTEM DESIGN

Long Polling vs WebSockets vs Server-Sent Events

Let's talk about three different ways to get real-time updates in web applications, because choosing the right approach can make a huge difference in your app's performance.

Repeatedly asking the server for updates (Regular Polling isn't great because it wastes resources. Imagine refreshing the app every few seconds for a message that might never come. Your battery drains, and the server gets overwhelmed with unnecessary requests.

When your web app needs to receive live updates from a server, you have three main options: Long Polling, WebSockets, and Server-Sent Events (SSE).

Long Polling is smarter than Regular Polling. Instead of constantly checking, the client makes a request and the server holds onto it until there's actually new data to send. Once the server responds, the client immediately makes another request. While this is better than regular polling, it still has drawbacks because you need to create new connections frequently, which involves sending extra headers and using server resources.

WebSockets are more sophisticated. They create a persistent, two-way connection between the client and server. This means both sides can send data to each other at any time without establishing new connections. WebSockets are great for applications that need frequent, real-time updates like chat apps or live gaming. However, keeping these connections open can waste resources if you're not sending data frequently.

Server-Sent Events (SSE) offer a middle ground. Like WebSockets, they maintain a persistent connection, but they only allow the server to send data to the client (one-way communication). If the client needs to send data back, it uses regular HTTP requests. SSE also offers automatic reconnection if the connection drops. However, this can cause problems in large applications through something called the "Thundering Herd" problem - if the server goes down and comes back up, all clients trying to reconnect simultaneously might overwhelm it.

Here's a comparison of the three approaches:

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NEWS

This Week in the Tech World

Amazon Launches Budget Store: Amazon debuts "Amazon Haul," a discount store with all items under $20, targeting Temu and Shein. Products ship directly from China with 2-week delivery and free shipping on orders over $25.

Bitcoin Hits Record High: Bitcoin surged above $91,000 for the first time, rising over 2% to $91,705. The cryptocurrency rallied following Donald Trump's election victory and October inflation data showing a 2.6% annual rate.

Musk's Trump Gains: Tesla's 39% stock surge since Trump's election victory adds $70 billion to Elon Musk's net worth. Musk, who spent $130 million supporting Trump, sees wealth rise to $320 billion.

Klarna Files for US IPO: Swedish buy now, pay later giant Klarna has filed for a US IPO. Recently valued at $15 billion, down from its $46 billion peak in 2021, Klarna aims to list in New York despite European exchanges' efforts.

Startup CEO “No humans needed for translation in 3 years“: Unbabel launches Widn AI translation service powered by its LLM Tower. CEO Vasco Pedro claims human translators won't be needed within three years due to AI advancements.

23andMe Cuts Jobs: Genetics company 23andMe announces 40% workforce reduction, cutting 200 jobs and ending therapeutics programs. Company reports declining revenue and seeks additional capital amid 75% stock drop.

Block Scales Back Crypto: Jack Dorsey's Block winds down its TBD crypto unit amid layoffs. Company maintains bitcoin holdings worth $630 million but shifts focus to business lending and Afterpay integration.

Meta Cuts EU Subscription Fees: Meta reduces Facebook and Instagram ad-free subscription prices by 40% in EU to comply with regulations. Monthly fees drop to €5.99 for desktop and €7.99 for mobile users.

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